1. Field of the Invention
The present invention relates to a coating material for suppressing detrimental reflections of light by the internal surface in optical parts comprising optical glass such as lens, prism and the like.
2. Description of the Prior Art
Internal reflection of a lens is explained below by taking up a lens for a camera as an optical part. FIG. 1 shows schematically a cross sectional view of a lens for a camera. "a" denotes a lens barrel, "b" an aperture and "c" lenses.
When an external light 1 enters from the front side of the lenses, there occur irregular reflections as shown by lights 2, 3 and 4 since the peripheral surfaces of the lenses are roughly ground and there are relatively large unevenesses and the peripheral surfaces are in a form of frosted glass.
For example, light 2 proceeds toward the rear portion of the lens and forms ghost, or flare to lower the contrast of images. A reflection light follows light path 4 enters an observing eye in front of the camera and lowers remarkably the commercial value because the peripheral surface appears to be bright.
The "internal reflection of an optical glass" in question in this invention refers to the phenomenon that such reflection lights as 2, 3 and 4 are generated.
An internal reflection suppressing coating material is a black coating material applied to the peripheral surface, and the enlarged cross section is shown in FIG. 2. 1 denotes an external light and 5 denotes a reflection suppressing coating which absorbs the external light and prevents its reflection.
Heretofore, as the reflection suppressing coating material, there have been used black coatings, but the internal reflection suppressing ability is very unsatisfactory. For the purpose of sufficiently achieving this function, it is preferable that the refractive index of the coating is the same as that of the glass of the lens to be coated and the coating sufficiently densely contains a light absorbing material.
However, the refractive index of optical glasses widely ranges from less than 1.5 to over 1.8 while the refractive index of coating material ranges at most from 1.5 to 1.6 because components of the coating are organic materials. In particular, in case of glasses of a high refractive index the internal reflection suppressing coating of the prior art cannot sufficiently absorb the light incident thereto and the internal reflection does not decrease.